Process for producing silver iodobromide photographic emulsion



United States Patent US. C]. 96-94 4 Claims ABSTRACT OF THE DISCLOSURE A process for the production of photographic silver bromo-iodide emulsions of enhanced sensitivity comprising the steps of preparaing an aqueous suspension of fine particles of silver iodide which possess an excess of silver ions and adding this suspension to an aqueous solution containing bromide ions.

The present invention relates to an improved process for the production of a photographic silver halide emulsion of enhanced sensitivity.

A photographic silver halide emulsion is usually prepared by the following steps: 1) the emulsification of silver halide particles and the first ripening therefor, (2) the removal of excessive salts, and, (3) the second ripening. In step 1), an aqueous silver nitrate solution is usually added to a solution containing dissolved therein a small amount of gelatin and a soluble halide to obtain emulsified silver halide particles and this procedure is generally called the single jet mixing method. The soluble halide to be employed may be a single kind of halide or a mixture of several kinds of halides, but the latter is suitable for obtaining an emulsion of a higher sensitivity. For example, it has been well known in the art that silver chloro-bromide has higher light sensitivity than silver chloride and silver iodo-bromide has higher light sensitivity than silver bromide.

Since the distribution of silver iodide in and among silver iodo-bromide particles has a large influence on light sensitivity, gradation and processing behaviour of the emulsion, various mixing methods have been proposed for controlling its distribution. For example, there are proposed (1) an inverse mixing method in which a halide solution is added into a silver nitrate solution, (2) a double jet mixing method in which a silver nitrate solution and a halide solution are added at the same time, and (3) a combined mixing method in which the above meihod is employed together with the successive mixing method. However, such methods will not always give a silver halide emulsion having sufliciently desirable properties.

Therefore, an object of the present invention is to provide a photographic silver halide emulsion for highsensitivity negative photographic elements.

Another object of this invention is to provide a novel process for forming silver iodo-bromide particles.

Still other objects of this invention is to provide a process for improving the characteristics of photographic, silver halide, light-sensitive elements.

The inventor has found that by preparing a suspension ice of fine particles of silver iodide and gelatin in a state of the pesence of excessive silver ions and adding the suspension into an aqueous solution of a bromide, a highly light-sensitive, silver iodo-bromide emulsion containing the fine particles is obtained.

For example, when an excessive proportion of an aqueous slver salt solution is added into an aqueous solution containing an iodide and a small amount of gelatin with stirring violently within a short period of time, a suspension (I) of fine silver iodide crystals is obtained. In this case the amounts of the silver ions and the iodine ions to be added are selected such that 1-2O mol percent of thus added silver ions are converted into silver iodide. Further, the gelatin to be employed in this invention is preferably of an inactive type. Thereafter, the resulting suspension is added slowly into an aqueous solution of an excessive amount of bromide with stirring to provide silver iodo-brornide particles (II). In this case, the fine crystals of silver iodide are continuously supplied together with silver ions and hence the silver iodide is distributed more uniformly among the particles and, by the conversion of crystals, the final silver iodo-bromide particles are formed. On the other hand, in a conventional single jet mixing method, iodine ions will be consumed completely in the earlier period of the addition of the silver ions and accordingly, the distribution of silver iodide among the particles of silver iodo-bromide will tend to become uneven. Moreover, although in a conventional inverse mixing method the distribution of silver iodide among particles may be more uniform than the case of the single jet mixing method, there is difficulty in the growth of crystals in such a method, which makes the method unsuitable for the production of high-sensitivity silver halide emulsions. Furthermore, the distribution of silver iodide among the particles by a conventional double jet mixing method may be uniform to some extent, but it is not always possible to obtain completely uniform distribution of silver iodide.

Further, according to conventional mixing methods, particles of silver iodo-bromide are formed by coprecipitation in each case. While, according to the process of the present invention, the distribution of silver iodide among particles can be made uniform as well as by using the silver iodide nuclei, silver iodo-bromide particles can be made which have properties different from those of the silver iodo-bromide particles prepared by conventional mixing methods.

In the case of preparing the silver iodo-bromide particles by the process of this invention, gelatin may be used, if necessary, and the gelatin may be an active type, although an inactive type is preferable, as mentioned above. Further, instead of gelatin, there may be employed in the process of this invention such synthetic high molecular materials as polyvinyl lactams, polyvinyl alcohols, and polyacrylamides.

After the formation of the silver iodo-bromide particles, the light-sensitivity thereof is increased by conducting the first ripening, the removal of excessive salts, and, then, the second ripening as in the case of conventional mixing methods. It is preferable to conduct the second ripening in the presence of a small amount of a sulfur compound, a noble metal salt and a reducing material. If desired, the silver halide emulsion may be applied to a suitable support with the addition of a light-sensitive dye, a stabilizer, a hardening agent, and a wetting agent to provide a photographic silver halide light-sensitive element.

The invention will further be explained by the following typical examples, although the invention shall not be limited to them in any way.

EXAMPLE 1 Solution I Potassium iodide g 6.64 Inactive gelatin g 10 Distilled water to make ml 400 Temperature C 65 Solution II Silver nitrate g 68 Distilled water to make ml 800 Temperature C 65 Solution II was added into Solution I within seconds with rapid stirring.

Solution III Potassium bromide g 58.8 Inactive gelatin g 18 Distilled water to make ml 600 Temperature C 65 Into Solution III was added 300 ml. of the mixture of Solution I and Solution II within 30 seconds with stirring. Then the system was stirred for 4 minutes and 30 seconds. This procedure was repeated 4 times to add the whole proportion of the solution mixture (I-l-II) into Solution III. Then, after the addition of 140 g. of gelatin, the system was ripened by heating for 12 minutes at 65 C. Thereafter, the thus obtained emulsion was diluted to 2200 ml. and set by cooling. The product was shredded into noodles and washed with water until the electric conductivity thereof became 1,200 ,u.V./CII1. After adjusting the pH thereof to 6.8 and the pAg thereof to 8.2, the thus prepared emulsion, corresponding to g. of silver nitrate, was mixed with 4 ml. of an aqueous solution of 0.01% of Na S O -5H O, as a sulfur sensitizer, and a mixture of 2 ml. of an aqueous solution of 0.01% of HAuCl -4H O and 2 ml. of an aqueous solution of 1% of KSCN, as a gold sensitizer. The mixture was then subjected to a second ripening by heating for 60 minutes at 55 C. The resulting silver halide emulsion was applied to an undercoated glass plate in a thickness of about microns and dried. The light-sensitive glass plate was exposed for second by means of a sensitometer having an intensity scale of 2600 K. and was developed in a developing solution of a D-76 type Metol-hydroquinone developer for 7 minutes at 20 C. The results of density measurements of the thus developed plate were as follows: the fog was 0.07; the exposure amount necessary for density 0.1 above fog was 0.12 c.m.s.; and the gamma was 0.91. The abovementioned emulsion layer obtained by the process of this invention had a good granularity. On the other hand, by a conventional successive mixing method in which Solution I and Solution III are mixed preliminarily and then Solution II is added into the solution mixture, the granularity was worsened and, further, a high sensitivity was not obtained. Furthermore, when Solution II was added to Solution III and, thereafter, Solution I was added to the solution mixture, the sensitivity was markedly reduced.

EXAMPLE 2 While in the compositions shown in Example 1 the total mole numbers of the halides were maintained constant,

4 the amount of the initially prepared silver iodide suspension was varied. The results obtained in this example by repeating the same procedure as in Example 1 are shown in Table 1.

Experiment Number EXAMPLE 3 Using the compositions shown in Example 1, Solution I was added with stirring to Solution II over a period of 1 minute and the whole amount of the solution mixture of Solution I and Solution II was continuously added with stirring to Solution III over a period of 20 minutes. The second ripening in this case was conducted using the gold sensitizer in an amount six times as much as in Example 1. The prepared emulsion corresponding to 10 g. of silver nitrate was mixed with 5 ml. of a methanol solution of 0.1% of 3,3-diethylthia-carbocyanine-iodide, as a color sensitizer, 12 ml. of an aqueous solution of 1% of 4-hydroxy-6-methyl-1,3,3a, 7-tetraazaindene, as a stabilizer, 12 ml. of an aqueous solution of 2% of mucochloric acid, as a hardening agent, and 1 ml. of an aqueous solution of 2% of polyoxyethylene sorbitan monolauric acid, as a wetting agent, and then applied to a film base of cellulose acetate and dried to 15 microns in thickness. It was confirmed by sensitometry that the thus prepared light-sensitive film has a longer linear portion of the characteristic curve, finer granularity, and better sharpness and clearness than a film of almost the same sensitivity prepared by a conventional successive mixing method.

What is claimed is:

1. A process for the production of a photographic silver halide emulsion which comprises preparing a silver iodide suspension by mixing an aqueous solution containing an iodide and a protective colloid with a solution containing a silver salt in such proportions that the silver iodide suspension contains an excess of silver ions, and adding said silver iodide suspension to an aqueous solution of a bromide, the amount of bromide being in excess of the silver ions, to form an emulsion containing silver iodobromide particles.

2. The process according to claim 1 wherein said pro portions are such that from about 1 to 20 mole percent of the added silver ions are converted into silver iodide.

3. The process according to claim 2 wherein said iodide is potassium iodide, said protective colloid is an inactive gelatin, and said silver salt is silver nitrate.

4. The process according to claim 1 wherein said aqueous solution of a bromide contains a small proportion of an inactive gelatin and said bromide is potassium bromide.

References Cited UNITED STATES PATENTS 7/1956 MacWilliam 96-94 8/1961 Luckey et al. 9668 OTHER REFERENCES J. TRAVIS BROWN, Primary Examiner 

